These measuring devices have, to the present point in time, most often been implemented in the form of pressure difference measurement transmitters, which comprise a pressure difference measuring transducer, which contains a pressure difference measuring membrane, which is contactable with a first pressure and a second pressure, wherein the first pressure counteracts the second pressure, so that the resulting deformation of the pressure difference measuring membrane is a function of the difference between the first pressure and the second pressure.
The conveying of the first pressure and of the second pressure from remote locations to the pressure difference measuring transducer usually occurs hydraulically, via oil-filled capillary lines. Due to the installation effort associated therewith, there have, for some time, been approaches to determine the pressure difference, wherein the first pressure and the second pressure are separately registered, the first pressure with a first pressure measuring transducer and the second pressure with a second pressure measuring transducer, and the pressure difference is determined based on the signals of the first and second transducers.
This principle is explained, for example, in “Hydrostatic Tank Gauging—technology whose time has come” (1990, ISA Services, Inc.).
For example, in 1992, Endress+Hauser offered and described a modular measuring apparatus for fill level measurement, “Liquid level transmitter Model DB40RL Sanitary Sensor deltapilot”. Such apparatus includes two pressure measuring transducers, whose PFM signals are transmitted to an evaluation unit, wherein the evaluation unit digitizes the PFM signals and, based on the digitized PFM signals, calculates the pressure difference.
U.S. Pat. No. 5,227,782 discloses a so-called Hydraulic Interface Unit (HIU) for performing bidirectional, digital communication with a plurality of pressure measurement transmitters, wherein the communication signals are superimposed on the energy supply signal.
U.S. Pat. No. 5,870,695 discloses another pressure difference measuring arrangement with remote sensor units, whose signals are transmitted electrically.
On the one hand, a measuring arrangement with a remote evaluation module is present, which is connected with two pressure measuring transducer modules, in each case via a cable. This leads to a large cabling effort if the evaluation module is positioned a large distance from the pressure measuring transducer modules. Moreover, due to disturbance resistance, it can be required that the two interfaces of the evaluation module with to the pressure measuring transducer modules be equipped with disturbance removal measures, since, otherwise, especially in the case of hardwired disturbances, malfunctions can occur. A reason for this is, for example, that a typical 4 . . . 20 mA device possesses an electrical current control OP, which controls the electrical current through a shunt resistor. If the capacitance between the housing potential and circuit ground is too high, in the case of line-coupled disturbances, an error current flows through this shunt resistor. This, in turn, has the result that the electrical current control no longer functions faultlessly.
In other measuring arrangements, a so-called slave measurement transmitter is connected with a master measurement transmitter, wherein the slave transmitter provides to the master measurement transmitter a first pressure measurement value, and the master measurement transmitter itself ascertains a second pressure measurement value and contains an evaluation unit to calculate the pressure difference. This solution has the disadvantage that the master measurement transmitter, which usually contains servicing elements and connection clamps for a supply and data line from or to the control system, must be mounted on a process opening, for example, at the floor of a tank in the so-called sump area. This site can be difficult to access, for example. In the case of other measuring locations, unfavorable conditions for an operator or for a computer can reign there, be it extreme temperatures, media emissions, condensate, vibrations or noise. This solution is, consequently, not suitable for all measuring locations.